There is an increased interest in non-contact systems that are able to visualize in real-time objects under a light obstructing barrier. The techniques are either relatively inexpensive, but are based on metal detection or can detect a broad range of materials, in particular soft materials, but are expensive and with limited range.
Ultrasound has been used in order to detect objects under clothes. In this technique, ultrasound is emitted towards a target, and the ultrasound echo is recorded. In a way similar to medical echography, all the different layers below the cloth are providing a weak echo, allowing reconstructing an image of the underlying object. This method has a quite limited range because of the strong attenuation of the ultrasound in the air. Moreover, and this is more critical, it has a very low resolution. In fact, there is a link between the attenuation and the resolution. Typically ultrasound in air is very strongly attenuated after a few meters for ultrasound frequencies above 30 kHz. This value corresponds to a resolution of approximately 1 cm. Therefore this technique has a limited range-resolution trade-off.
A problem often associated with the detection of said objects is the fact that the detection techniques often have a limited range, and can only detect said objects situated at proximity of the detection system (for example X-ray or computerized tomographic systems).
In the prior art, buried mines have been detected using a combination of sound and laser light. Most use a vibrometer, which is expensive and require the optical head to be in close contact with the object that is measured. Another technique uses two fibers in order to reduce the problem. However, this technique needs close contact to the target. The technique does not provide parallel detection and real-time image (scan of the laser). In addition, due to the use of ultrasonic reflection, the prior art technique has a limited range-resolution trade-off. For example, a choice must be made between ˜1 cm resolution with limited range (apparently less than 1 meter) or 5 meters range with 10 cm resolution. Moreover, the use of ultrasonic pulses has another disadvantage: one may hear sound at the pulse frequency.